Zinc/air depolarized cells are typically in the form of miniature button cells which have particular utility as batteries for electronic hearing aids including programmable type hearing aids. Such miniature button cells typically have a disk-like cylindrical shape of diameter between about 4 and 20 mm, typically between about 4 and 16 mm and a height between about 2 and 9 mm, preferably between about 2 and 6 mm.
The miniature zinc/air button cell typically comprises an anode casing (anode can), and a cathode casing (cathode can). The anode casing and cathode casing are in the shape of separate cans each having a closed end, an open end, and integral side walls extending from the closed end to the open end. The anode casing is fitted with an insulating seal ring which tightly surrounds the anode casing side wall. Anode material is inserted into the anode casing. Air diffuser, electrolyte barrier material, and cathode assembly are inserted into the cathode casing adjacent air holes in the cathode casing. The open end of the cathode casing is then typically pushed over the open end of the anode casing so that the cathode casing side walls overlaps essentially the entire anode casing side wall with insulating seal therebetween. The anode and cathode casing are then interlocked in a second step by crimping the edge of the cathode casing over the insulator seal and anode casing side wall. During the crimping procedure (or in a separate step) radial forces are also applied to the cathode casing walls to assure tight seal between the anode and cathode casings.
Representative zinc/air button cells showing the conventional configuration with separate anode and cathode casings each in the form of “cans” having a closed end and opposing open end are shown, for example, in U.S. Pat. Nos. 3,897,265; 5,279,905; and 6,830,847 B2. As seen in each of these representative patents, the anode can and cathode can each have a closed end with opposing open end and integral side walls therebetween. After cell contents are inserted, the cathode can side wall is pushed over essentially the entire anode can side wall with insulating sealing ring therebetween. The peripheral edge of the cathode can side wall is crimped over the peripheral edge of the anode can side wall with the insulating sealing ring therebetween. Such configuration is designed to produce a durable cell with reduced chance of electrolyte leakage.
The anode casing of zinc/air button cells may be filled with a mixture comprising particulate zinc. Typically, the zinc mixture contains mercury and a gelling agent and becomes gelled when electrolyte is added to the mixture. The electrolyte is conventionally an aqueous solution of potassium hydroxide. The closed end of the cathode casing (when the casing is held in vertical position with the closed end on top) may have a flat raised portion near its center. This raised portion forms the positive terminal and typically contains a plurality of air holes therethrough. In this design, the cathode casing closed end also typically has an annular recessed step which surrounds the raised positive terminal. Alternatively, the closed end of the cathode casing may be completely flat across its diameter, that is, without any raised portion at its center. In such design the central portion of such flat area at the closed end of the cathode casing typically forms the cell's positive terminal. In either case, the closed end of the cathode casing of button zinc/air cells is punctured with one or more small air holes to allow air to enter the cell. Such air then traverses an air diffusion layer (or air diffuser) in order to reach the cathode disk.
The air diffuser material is normally composed of one or more sheets of air permeable paper or porous cellulosic material. Such permeable paper or porous cellulosic material allows incoming air to pass uniformly to the cathode assembly and also may serve as a blotter to absorb minor amounts of electrolyte which may leak into the air inlet space. The air diffuser is normally placed uniformly within the air inlet space (plenum space) between the closed end of the cathode casing and cathode assembly. The air diffuser material fills such air inlet space and covers the air holes in the closed end of the cathode casing. Commercial button size zinc/air cells which are commonly used in hearing aid devices may have only one air hole or may have a plurality of small air holes, for example, between 2 and 6 air holes and even more depending on cell size.
Catalytic material typically comprising a mixture of particulate manganese dioxide, carbon, and hydrophobic binder can be compacted into a disk shape forming a cathode disk within a cathode assembly. The cathode assembly with cathode disk therein can then be inserted into the cathode casing over the air diffuser on the side of the air diffuser that faces away from the air holes. Typically a cathode assembly is formed by laminating a layer of electrolyte barrier material (hydrophobic air permeable film), preferably Teflon (polytetrafluoroethylene), to one side of the catalytic cathode disk and an electrolyte permeable (ion permeable) separator material to the opposite side of the catalytic cathode disk. The cathode assembly with cathode disk therein is then typically inserted into the cathode casing so that its central portion covers the air diffuser and a portion of the electrolyte barrier layer rests against the inside surface of the step. The cathode disk in the final cell contacts the cathode casing walls around its perimeter.
The cathode assembly may have a flat or domed shape. The flat cathode assemblies are preferred, since they are easier and more economical to fabricate. Representative zinc/air button cells with flat cathode assemblies are shown in U.S. Pat. Nos. 5,279,905; 6,602,629 B1; and U.S. Pat. No. 6,830,847 B2.
If the cell is not adequately sealed, electrolyte can migrate around the catalytic cathode assembly and leak from the cathode casing through the air holes. Also electrolyte leakage can occur between the crimped edge of the cathode can and insulator if this area is not tightly sealed. The wall thickness of commercial zinc/air button cells are typically greater than about 6 mil (0.152 mm), for example, between about 6 and 15 mil (0.152 and 0.381 mm). The potential for leakage is greater when the anode casing and cathode casing is of very thin wall thickness, for example, between about 2 and 5 mil (0.0508 and 0.127 mm). Such low wall thickness is desirable, since it results in greater internal cell volume. But there are limits to how thin the anode and cathode can side walls can be made without sacrificing cell integrity.
After the cell is assembled a removable tab is placed over the air holes on the surface of the cathode casing. Before use, the tab is removed to expose the air holes allowing air to ingress and activate the cell.
It is desired to improve cell capacity of zinc/air button cells by increasing the available anode can internal volume for insertion of anode material therein for any given size cell.
Specifically, it is desired to increase the available anode can internal volume for insertion of anode material therein for any zinc/air button cell of given overall cell diameter and height.
It is desired that the overall cell construction result in a durable cell having a tight seal which resists electrolyte leakage.